Various elongated objects, among which nano-objects such as nanowires or nanotubes, are currently used for many applications. Nanowires are used for example to manufacture piezoelectric devices, such as capacitive deformation sensors, or optical devices such as light-emitting diodes or photovoltaic devices.
There are two major methods for manufacturing nano-objects. The first method consists in growing the nano-objects in a solution, without a substrate. In this case, the nano-objects are dispersed in the solution. The second method consists in growing the nano-objects using a substrate, according to the same orientation with respect to the surface of the substrate.
Among the techniques of growing nano-objects, mention can in particular be made of molecular beam epitaxy (MBE), and vapour phase epitaxy using for example molecules of the metal organic type or hydrides as gas source, respectively called MOVPE for “Metal Organic Vapour Phase Epitaxy” and HVPE for “Hydride Vapour Phase Epitaxy”.
The nano-objects can also be obtained by other methods such as liquid phase chemical growth or top-down approaches based on lithography and etching methods.
The nano-objects are anisotropic, i.e. they have one or several physical properties that vary according to the orientation considered. Therefore, it is necessary for certain applications to orient the nano-objects in a desired orientation. Indeed, when the physical characteristic considered is oriented, it is necessary to orient this characteristic of all of the nano-objects in the same orientation. Indeed, nano-objects that are arranged in the same orientation but head-to-toe, i.e. in opposite directions, can have their responses to an excitation be cancelled rather than added together. This is the case for example in the field of piezoelectricity.
A known method of orienting nanowires is the Langmuir-Blodgett method, shown in FIGS. 1A and 1B. However, this method can be applied only to nanowires 101 dispersed in a solution 102. Consequently, in order to orient nanowires 101 manufactured using a substrate, it is first of all necessary to disperse them in the solution 102. Then, after the adding of chemical ligands, the nanowires 101 in suspension in the solution 102 rise to the surface and form a layer of nanowires 101. The density of nanowires 101 on the surface of the solution 102 is controlled by the displacement of hydrophobic barriers 103 which exert a pressure on the layer of nanowires 101. The nanowires 101 are then transferred onto a support 104, with the latter being plunged in the solution 102 then removed from the solution 102. A first disadvantage of this method is that these technological steps are expensive and are time consuming.
Moreover, on the support 104, the nanowires 101 are globally parallel to one another but the direction of their crystallographic axis is not always the same. FIG. 1B is a top view of the surface of the solution 102 and shows the loss of orientation of the nanowires 101, with the crystallographic axes being represented by arrows 105.
With this method, the nanowires 101 also tend to overlap and to form bundles, which can hinder, according to the target application, the operation of the final device wherein the support 104 is arranged. As such, the Langmuir-Blodgett method makes it possible to assemble nanowires 101 only on small surfaces, up to 16 cm2, because for larger surfaces, the inhomogeneity of the pressure exerted solely on the edges of the layer of nanowires 101 would be excessively high.